Television projector tube



DeC- 10, 1946 D. B. LANGMUIR ET AL TELEVISION PROJECTOR TUBE Filed Aug.ze, 1943 Patented ee. l.,

assignors to Radio vC'rporatlonl ofv 4 America, a corporation ofDelaware AppllcationAu'gust 26, 1943, Serial No. 500,030 'fv ,v

The present invention relates to light valves,

and more particularly to light valves for use in j the reproduction oftelevision images,

In most television receivers, particularly: those i' used in the home,the television image is produced on a fluorescent or luminescent targetor screen positioned on the end wall of a cathode ray tube. The targetvor screen has the characteristie that, when bombarded by an electronbeam, a fluorescent or luminescent effect is produced, with the resultthat light is emitted from the bombarded area. Accordingly, when acathode ray beam is scanned over the screen surface, and when thecurrent intensity of the 'cathode ray beam is varied in accordance withthe received television signals, e, light image will result. Such amethod of producing television images is entirely satisfactory whenlarge images are not required, and when the number of persons viewingthe image is limited. l

There are conditions, however, when it is desirable to produce largetelevision images on a screen suitable for viewing by a large audience,and under these conditions the use of a direct viewing cathode ray tubeis impractical.

Various devices, generallyin the form of a, projection Kinescope, arewell known to those skilled in the art for use in the production oflarge screen television pictures, and such devices include a. highvoltage cathode ray tube of special design which is capable of emittinga large amount of light sufcient, when projected through an appropriateoptical system, to produce a satisfactory large screen picture. Indevices of this nature, the cathode ray beam must generate the powerwhich is later converted into light, and if 7 claims.' (ci. 11s- 7.5)

l to control the passage of light through a'normally opaque medium,together with means for reconf dltioning the medium or valveso. that itmay brilliant images of large dimensions are to proy duced, a relativelylarge amount of powel` is required.

In order to be able to utilize a separate source of light so that thecathode ray beam 'would not have to generate the power to be convertedinto light, but would merely control the light from a constant source ofhigh intensity, various systems have been devisd in which means areprovided which operate in response to a cathode ray beam to control thetransmission of light from the source to the viewing screen. In suchdevices some form of a light valve must be used in order to control andmodulate the amount .of light which is permitted to pass from -thesource to the viewing screen. 1

The present invention is therefore concerned with such a lightv valvewhich will respond to current modulations of a cathode ray beam in orderproperly respond to the current intensity of the scanning cathoderaybeam during the next scanning cycle. i

Light valves for accomplishing these results are shown and described inthe patents to Donal Nos. 2,290,581 and 2,290,582, granted on July 21,1942.' In each of these patents aspecial form of a cathode ray tube isvshown 'which includes a scanningcathode ray beam and a target surface.Positioned adjacent thetarget surface is a medium whichcontains a large"numberof particles for normally preventing the transmission of lightthrough the medium- When an electrostatic potential is applied across-the medium, the particles are caused to orient themselves in such amanner that light projected parallel to the direction of theelectrostatic iieldmay pass therethrough. The degree of orientation is afunction .of the electrostatic field intensity and the currentintensityof the scanning cathode ray beam, with the result that themedium acts asa variable and controllable-light valve for modulating-the light lbeam projectedthrough the medium.

The electrostatic field orpotential diilerence to which the mediumk issubjected, as stated above, is a function of the current intensity ofthe scanning cathode ray beam. After the cathode ray beamxhas traverseda predetermined portion of the target or screen surface adjacent themedium, the electrostatic field persistsl for apredetermined length oftime, and some provision must -be made for removing the produced eldprior to the next scanning cycle in order that a newileld may beestablished in accordance with` the current intensity of the cathode raybeam` as long as'possible in order to obtain high optical emciency. Whena medium is used in which opaque particles are in suspension, it-is notconvenient to rely upon leakage for the removal or dissipation of theelectrostatic fields produced by the scanning operation, since the.resistance of the wall surfaces retaining the medium is generally toohigh. Furthermore, if leakage is relied upon, the electrostatic iield'condition and the light valve action do not persist to their fullextent for an appreciable portion of a eld cycle.

with the result that the optical emciency of the light valve is reduced.It is, therefore, desirable that some means be provided for removing theproduced electrostatic field just prior to the establishment of a neweld in accordance with the new current conditions of the scanningcathode ray beam. When such a result is accomplished, the light valvemay then be operated at near its optimum efilciency.

In the present invention the light valve cathode ray tube includes twoelectron gun structures for producing separate and independent cathoderay beams. One of these beams is modulated by the received video orpicture signals to produce variable electrostatic charge potentialsacross the light valve structure, and the other cathode ray beam, whichis generally of constant current intensity, is used to destroy or removethe produced electrostatic field just prior to the re-establishment of anew field by the modulated beam. In order that the two cathode ray beamsmay have opposite effects on the light valve, i. e., one to produce anelectrostatic field for permitting the transmission of light, and theother to destroy or remove that field, the two beams cannot be identicalin all respects. The two beams and their relation to the target screenmay differ from each other in different respects in order to produce thedesired result. In one instance, the two beams may be operated atdifferent velocities, or in another instance, the two beams may beprojected against the target or screen surface at different angles. Itis also possible to employ a combination of intensity difference andimpact angle difference. In each case, however, the desired result isaccomplished by the resulting secondary electron emissive effects'produced at the target surface.

In accordance with the present invention, the modulated cathode ray beamwhich operates to produce the desired electrostatic field is directedagainst the target surface in such a manner and at such a velocity thatthe number of secondary electrons produced is less than the number of'arriving or impinging primary electrons. Under these conditions thetarget surface is caused to accumulate a negative charge, or, in otherwords,

- its potential is driven in a negative direction in accordance with thecurrent intensity of the impinging or scanning cathode ray beam. Thisaction establishes a variable electrostatic field in accordance with thebeam modulations, so that the light valve may modulate a light beamprojected therethrough to produce the desired image.

An interval of time later (corresponding to less than one field cycle),the second cathode ray beam is directed against the target surface atsuch a velocity and at such an angle that the number of' secondaryelectrons produced is considerably in excess of the number of arrivingor impinging primaryl electrons. The second cathode ray beam isgenerally of constant current intensity. Since more electrons are drivenfrom the target surface than arrive thereat, the potential of thesurface is altered in a positive direction, with the result that thepreviously produced electrostatic field is eradicated. The light valveis then conditioned for a subsequent reestablishment of an electrostaticfield in accordance with the new modulated conditions of the scanningcathode ray beam.

IWhen an electronic light valve is operated as described above and inaccordance with the present invention, a large screen television picturemay be produced due to the modulation of the 4 amount of light permittedto pass through the light valve. Furthermore, under these conditions theeffect of the light valve may be caused to persist for almost one entiretelevision field cycle, withv the result that the optica1 eiliciency ofthe system is materially enhanced.

It is, therefore,` one purpose of the present invention to provideanelectronic light valve of improved efficiency.

Another purpose of the present invention resides in the provision of anelectronic light valve for use, for example, in the production oftelevision images in which the effect of the light valve may be causedto persist for substantially an entire television field cycle.

vStill another purpose of the present invention resides in the provisionof an electronic light valve for controlling or modulating the amount oflight that is permitted to pass therethrough in accordance with thecurrent modulations of a scanning cathode ray beam.

A still further purpose of the present invention resides in theprovision of an electronic light valve wherein two separate cathode raybeams are provided, one of which is effective to produce the desiredelectrostatic field, and the other of which is effective to cancel oreliminate the produced field.

Another purpose of the present invention resides in the provision of anelectronic light valve in which an electrostatic field or potentialdifference is produced as the result of a scanning operation of onecathode ray beam, the intensity of which is modulated in accordance withthe potential variations, and the destruction of the producedelectrostatic field by another cathode ray beam of substantiallyconstant current intensity.

A still further purpose of the present invention resides in theprovision of an electronic light valve in which two separate cathode raybeams are used, each of which produces different secondary electronemissive effects on a target surface to alter the potential of thesurface in one direction or another.

Another purpose of the present invention resides in the provision of anelectronic light valve in which two cathode ray beams are used, thevelocity and/or impact angle of one beam being such that a secondaryelectron emission ratio less than unity is produced, and in which thevelocity and/or impact angle of the second beam is such that a secondaryelectron emission ratio in excess of unity is produced.

Still other purposes and advantages of the present invention will becomemore apparent to those skilled in the art from the following detaileddescription, particularly when considered in connection with thedrawing, wherein Figure 1 shows one form of the present invention; and

Figures 2 and 3 are curves used in explaining the theory of operation ofthe present invention.

Referring now to the drawing, and more particularly to Figure 1 thereof,there is shown a cathode ray tube I0 and a source of light I2. 'I'hecathode ray tube is provided with light transmitting walls I4 and I6,both of which are preferably optically flat and made of uniform materialso that no light distortion results. The wall I4 may be made of glass orsimilar transparent material, whereas the wall I 6 is preferably made ofmica and is, in fact, the target surface against which cathode ray beamsare directed. It is not necessary that the target .or wall I6 be made ofmica, but it should be composed of high electrical resistance materialhaving the desired quality of being optically transparent, and havingsufcient mechanical strength to withstand the hydrostatic or vaporpressures to which it is subjected. Even though the interior of thecathode ray tube I is exhausted to a very low pressure, it hasbeen foundthata sheet of mica, even when reduced to a few thousandths of an inchin thickness, is suiiiciently strong to withstand the pressures exertedthereon.

Since the target I6 forms one wall of the cathode ray tube, it is sealedto the glass envelope of the tube by means of a vitreous material I8.The coeiicient of expansion of the material is so chosen that a goodbond maybe maintained between the envelope of the cathode ray tube andthe sheet of mica I6. One manner in which the mica may be sealed to theglass envelope of the cathode ray tube I0 by the vitreous material i8 issuggested in the specification of Patent No. 2,290,581, referred toabove.

'Ihe end of the cathode ray tube adjacent the mica target I6 is providedwith a compartment or container in which is located a suspending mediumor liquid 20. The end 22 of the reservoir is preferably made ofoptically fiat glass or similar transparent material, in order that anoptical image may be passed therethrough without distortion.

The suspending medium or liquid 20 that is contained in the reservoirmay be any liquid having the desired characteristics as regardselectrical resistance, transparency, vapor pressure and viscosity. 'I'hesuspending medium or liquid preferably has very high electricalresistance and transparency, and low vapor pressure and viscosity. Anumber of suitable materals may be used as the suspending medium orliquid, these materials including liquids such as n-amylsebacate,ethyl-hexyl-phthalate, ethyl-hexylacetate and tetrabromoethane.

Suspended within the liquid or suspending medium is a large number ofilat particles or platelets 24. It has been found that small particlesof graphite of a size larger than colloidal may be used, or commercialaluminum foil having a thickness less than 0.5 micron which has beensubdivided into particles that are very thin compared to their otherdimensions may satisfactorily be used. Other light opaque particleshaving the proper characteristics may also be suspended by the liquid orsuspending medium 20.

Since the particles are slightly larger than colloidal, they will notremain permanently in suspension, with the result that some means shouldbe provided for preventing precipitation of the particles. For thispurpose an agitator may be provided, as suggested in the above mentionedPatent No. 2,290,581, and, if desired, a heating element may be providedreducing the viscosity of the suspending medium, as suggested in thesame patent.

It has been found that when the liquid 20 and the suspended particles 24are subjected to an electrostatic field, the particles are caused toorient themselves in a plane parallel to the impressed electrostaticeld, and if light is directed through the medium parallel to theelectrostatic field, the particles, due to their orientation, do notintercept an appreciable amount of the light. I n the absence of such anelectrostatic fieid, however, the particles assume a random orientation,and by reason of the fact that 6 the particles are opaque, light cannotbe passed through the liquid or suspending medium. Accordingly, themedium may operate as a light valve to permit or prevent, and thereby tocontrol, the transmission of light therethrough.

For impressing the desired electrostatic field upon the liquid 20, themica target 'I6 operates as one electrode, and for the other electrodean exceedingly thin film of metal 26 is deposited, preferably on theoutside surface of the container or reservoir wall 22. This metal,which, for example, may be gold or platinum, is applied as a conductinglayer over the surface of the end wall 20, and since it must conduct ortransmit light, should be exceedingly thin. The metal or conductingsurface may be applied by any desired method known to those skilled inthe art, such as by sputtering or by vapor condensation.

For projecting substantially parallel light rays through the cathode raytube and through the suspension liquid 20 from the light source I2, alens system 28 is provided. As stated above, the rays should passsubstantially normally through the plane of the suspension medium 20.Since the suspension medium 20 and the particles 24 contained thereinoperate as a light valve, a second lens system 30 is provided forprojecting the modulated light beam upon a screen or observation surface32.

The cathode ray tube l0 also includes two electron gun structuresrepresented generally at A and B. Gun structure A includes cathode 34,control electrode 36 and first anode or accelerating electrode 38. Thesecond electron gun structure B includes a cathode 40, a controlelectrode 42 and a rst anode or accelerating electrode 44. The secondanode or accelerating electrode (common to both gun structures A and B)is preferably in the form of a conducting coating 46 on the insidesurface of the tube, and may be in the form of a carbonaceous layer orfilm produced as a result of a deposit of aquadag thereon. Naturally,the conducting coating is not present on the surfaces through whichlight is projected.

For applying operating potentials to the electron gun structures, asource` of potential 50 is provided which is represented schematicallyas a battery. Furthermore, for causing the desired deflections of thecathode ray beams produced'by each of the gun structures A and B,deflecting yokes 52 and 54, respectively, are provided. These deiiectingyokes may be energized from appropriate horizontal and verticaldeflection generators represented schematically at.56. Since thedeflection sensitivity and the angle at which the cathode ray beams aredirected against the target I6 is different for each of the two beams,separate deflection generators would, in most cases, be required. Thepresent invention is not concerned with the specific deflectiongenerators used, and since such generators and defiecting means are wellknown to those skilled in the art, further discussion of this apparatusis believed to be unnecessary,

The second anode 46 and the conducting layer 26 are preferably connectedtogether, and are, in turn, connected to the positive terminal of thesource of potential 50. The other elements of the gun structure areconnected to potential sources negative with'respect to the potential ofthe second anode, and the potentials of these electrodes are determinedin accordance with the desired aplauso 7 focal conditions of theproduced cathode ray beams andthe desired velocity of the beams.

Since the cathode ray beam generated by the cathode ray gun B may be ofuniform current intensity, the control electrode 42 of the gun structureB is connected to a point negative with respect to its associatedcathode 40, and the current intensity of the beam produced by this gunmay naturally be controlled by an adjustment of the potential of thecontrol electrode 42 relm ative to the cathode 40. Since thecathode raybeam generated by the gun structure A is preferably modulated by voltagevariations such as, for example, the video signals of a televisionreceiver, the control electrode 36 of this gun structure is connectedtoa point negative with respect to the cathode 34, and in thisconnection is included a source of modulating potentials representedschematically at 56.

The patents to Donal referred to above show an electronic light valve inwhich a modulated cathode ray beam is used for varying the lighttransmitting characteristics of the liquid in which a multiplicity ofparticles are suspended. A similar provision is made in the presentinvention, and for this purpose the cathode ray beam produced by the gunstructure A and modulated by the source of potentials 58 is provided.The present invention also includes a cathode ray beam generated by thegun structure B for removing or obliterating the electrostatic fieldproduced by the cathode ray beam from electron gun A. As explainedabove, the production and the cancellation of the electrostatic chargesare produced as a result of secondary electron emissive effects at thetarget surface I6.

For the purpose of explaining the operation of the present invention,reference is now made to the curves shown in Figure 2. This figure showstwo curves, 60 and 62, in which the ordinate represents secondaryemission ratio, While the abscissa represents electron beam velocity.

It will be assumed that the potential of the second anode 46 and thepotential of the conducting film 26 is that represented by point f inFigure 2. If the cathode potential of electron gun A is at point a, thenthe produced secondary electrons will be as represented by curve 60.Furthermore, if the cathode potential of the electron gun B is chosen tohave a value b, then the produced secondary electrons will be asrepresented by curve 62. If all of the factors and parameters, includingthe angle of incidence of the two electron beams, are identical, the twocurves 60 and 62 will be identical but will be displaced, as shown inFigure 2, by reason of the difference in the potential of the cathodesof electron guns A and B.

Since the conducting surface 26 is at a potential represented at f, itmay be assumed that before any scanning operation takes place the targetsurface I6 will also be at the same potential. If 'now the targetsurface I6 is scanned by the cathode ray beam produced by gun structureA, the number of secondary electrons produced at the target surface I6will be considerably less than the number ofarriving or impingingprimary electrons, as represented by the curve 66, with the result thatthe potential of the scanned portion of the target surface I6 willchange in a negative direction, and if the scanning operation persists,the potential of the target surface I6 will change from point f to pointd along curve 60. Continued scanning bythe electron beam from gun A willnot result in any further change in a negative direction in thepotential of the target surface I6 because, if the potential oi' thesurface of target I6 were more negative than point d, a number ofsecondary electrons would be produced in excess of the arriving primaryelectrons, which would bring about a change in a' positive direction.Accordingly, the extent of the change of the potential of the targetsurface I6 in a negative direction is limited to point d, the secondcross-over point of the curve 60.

It will now be assumed that the scanning operation of the electron beamfrom gun A is discontinued, and that the target surface I6 is scanned bythe cathode ray beam from the electron gun structure B. Since thecathode potential of the gun structure B is assumed to be at potentialb, the effective velocity of the cathode ray beam at the targetelectrode will be such that the number of produced secondary electronswill be in excess of the number of arriving primary electrons, asindicated by the intersection of curve 6'2 with potential point d inFigure 2. Under these conditions, more electrons will be emitted fromthe target surface I6 than arrive thereat, with the result that thepotential of the surface will change in a positive direction along curve62, and continued scanning by the cathode ray beam produced by gun Bwill cause the surface of the target I6 to reach a potential e of Figure2. However, the potential of the surface cannot go beyond thatrepresented at e since at this point the number of produced secondaryelectrons equals the number of arriving primary electrons.

If the target surface is again scanned by the cathode ray beam producedby gun structure A, the potential of the target surface I6 will bechanged in a negative direct from e to d as a maximum along curve 60.Accordingly, it may be seen that with alternate scannings of the cathoderay beams produced by gun structures A and B, the potential of thetarget electrode I6 may be caused to vary between points e and d asextreme limits.

In the operation of the device for the production of television images,the current intensity of the cathode ray beam produced by the gunstructure A is modulated in accordance with the received video signals.This current modulation aiects the extent to which the potential of thesurface of the target I6 is driven in a negative direction, and if thecathode ray beam is caused to scan the target surface I6, and issimultaneously current modulated by the received picture signals, anelectrostatic charge image may be produced on the target electrode.Since the target electrode is composed of an insulating medium, a chargeimage may be retained thereon, and the difference of potential (andresulting electrostatic field) between the elemental areas of the targetsurface I6 and the conducting iilm 26 causes different orientationeffects upon the particles 24 contained within the suspension medium2.0. These different orientation effects cause a variation in themodulation of the light transmitted through the medium along lateraldimensions thereof, with the result that an optical image of theelectrostatic charge image is produced at the viewing screen 32. Thiselectrostatic charge condition will persist for a length of timedepending upon leakage resistance and other factors, and for besteiiiciency of operation, as explained above, it should persist for atleast the major portion of one eld cycle.

The electron beam produced by the gun structure B is of sui'cientcurrent intensity to always bring the potential of each elemental areaof the surface of the target electrode I6 to its most positivepotential, i. e., to the potential represented at e in Figure 2.Accordingly. vafter the surface I6 has been scanned by the electron beamfrom gun structure B, the potential of each element of the surface ofthe target I6 is always brought to the same datum potential level. Theextent to which elemental areas of the surface of target I6 are chargedin a negative direction by the cathode ray beam from the gun structure Adepends upon the current intensity of this cathode ray beam asdetermined by the modulations produced by the applied modulatingpotentials.

Since it is desirable that the electrostatic charge image be retainedfor a large percentage of one television field cycle, the scanningoperations as produced by the two cathode ray beams are slightly out ofphase with the scanning operation produced by gun B. preceding slightlythe scanning operation produced by gun A. When such aphasal conditionexists, the potential of each elemental area of the target surface I6 isbrought to the datum level e by the cathode ray beam from electron gun Bjust prior to the establishment of a new electrostatic condition orpoten-.-

tial at that elemental area by the cathode ray beam from electron gunstructure A.v

When the electronic valve is operated in this manner,` satisfactorytelevision images may be projected on a viewing screen 32, and theoptical elciency of the light valve is at substantially electron gunstructure A and the cathode ray beam produced thereby is directed towardthe target surface I6 at a less acute angle than is the beam from gunstructure B. Since the cathode ray beam from gun structure B strikes thetarget surface I6 at a more acute angle, a larger numbeiof secondaryelectrons will be produced, and for cancellation of the charge image thenumber of produced secondary electrons should exceed the number ofarriving primary electrons.

To explain the theory of operation of the present invention when thecathodes of the two sun structures are at the same potential, referenceis now made to the curves shown in Figure 3. In this figure, cu'rve 64represents the second electron response characteristics of the targetsurface I6 when scanned by the cathode ray beam produced by the electrongun structure A, and similarly, curve 66 represents the secondaryelectron response characteristics of the target surface I6 when scannedby the cathode ray beam generated by theelectron gun structure B.

Since the cathode ray beam from the. gun structure B is directedagainst` thetarget surface I6 at a more acute angle than is the cathoderay beam from gun structure A, a largernum'ber of secondary electronswill be produced, and, as a result, the first cross-over'point l ofcurve. 66 occurs at a lower beam velocity than theilrst crossover pointm of curve 64, and also the second cross-over point oof curve 66 occursat a higher velocity than the second cross-over point' n of curve 64.'I'he curves both start atl the same optimum value. In order that aminimum dif- .i

ferential potential may exist between the'target surface I6 and theconducting illm 26 when the elements of the screen I6 are in adischarged condition, it is preferable thatv the potential of the secondanode 46 and that of the conducting lm 26 be at a value such asrepresented at e in Figure 2, rather than at f.

For the operation of the system as described y above.. it .is necessarythat the cathodes of the two electron gun structures be at considerablevariance insofar as their relative potentials are concerned. It has beenfound that such a condition is not absolutely necessary, and, in fact,`

the cathodes of the twov gun structures may be operated at identicalpotentials. When such is the case, the beam velocities of the twocathode ray beams are similar. and in order to producel thedesired'eiects, some means other than beam velocity must be relied uponfor producing different secondary electron emissive effects at thetarget electrode I6. To produce the desired different secondary electronemissive effects, the two electron beams may be directed against thetarget electrode I6 at different angles. For best results and toeliminate undesired shading eiects, it is preferable that the twoelectron gun structures lie in a planeparallel to the vertical scanningdirection, with the electron beam of one gun structure directed towardthe target surface at a more acute angle than the other beam.

Since the angle of incidence of the two cathode ray beams is different,a different secondary electron emission effect will result, since it isknown that when the impinging electronsy strike normally to a surfacethey emit fewer secondaries than when they strike at a glancing anglewith respect to that surface. Inasmuch as it is desired that the numberof produced secondary electrons be less than the number of arrivingprimary electrons for the modulated cathode ray beam that is to producethe electrostatic charge,

pointk since, in accordance with the above. as-

sumptions, the cathodes of the two electron gun structures are operatedat approximately the same potential. In explaining the'operation of thepresent in- ',vention with the cathodes of the two gun strucv tures Aand B at approximately the same potential, it will be assumed that thepotential of ture A, the potential of the surface'of the target I6 willchange in a negative direction along curveV 64 from o tov n, due to thefact that at this effective beam velocity, the number of producedsecondary electrons is less than the number of arriving or impingingprimary' electrons. The surface ofthe target I6 will, therefore, bedriven in a negative direction with point n as the limit, since, asexplained above in connection with Figure 2, continued'scanning of thetarget surface I6 with the cathode ray beam from electron gun structureA will not produce any further change in the potential of the surface ofthe target I6. If then scanning by the cathode ray beam from electrongun structure A is discontinued and the target surfacel I6 is scanned bya cathode ray beam generated by the electron gun structure B, the numberof secondary electrons produced for each arriving primary electron willbe considerably in excess of the ratio one-to-one, as indicated by theintersection of curve 66, and the vertice.

line n of Figure 3. Since the number of secondsince point o correspondsto the second crossover point of the secondary electron emission ratiocurve,

When the present invention is used for the production of televisionimages with the cathodes of the two gun structures at substantially thesame potential, variations in the change of the potential of the targetsurface I6 in a negative direction are produced by.variations in thecurrent intensity of the scanning cathode ray beam. In order that thecathode ray beam maygbe current modulated, a source of modulatingpotentials is applied to the control electrode 36 of the electron gunstructure A in a manner described above. Furthermore, in operating thedevice under these conditions, the scanning of the target surface by thetwo cathode ray beams is slightly out of phase, so that the cathode raybeam from the electron gun structure B scans a predetermined portion ofthe target surface I6 just prior in point of time to the scanning ofthis same portion by the electron beam produced by electron gunstructure A. When this phasal condition exists, the producedelectrostatic charge on the target surface I6 is retained for almost anentire television field cycle in order that maximum optical efficiencymay be obtained. Furthermore, by reason of the secondary electronresponse characteristics of the target surface I6 when scanned by thecathode ray beam from gun structure B, and by reason of the fact thatthe current intensity of this electron beam may be relatively' heavy,all portions of the surface of the target area I6 are brought to thesame datum potential corresponding to point o in Figure 3 just prior tothe re-establishment of a new potential by the modulated cathode raybeam from electron gun structure A.

The extent to which individual elemental areas of the target surface I6are driven in a negative direction from a potential corresponding topoint o in Figure 3 is a function of the current intensity of themodulated cathode ray beam, with the result that an electrostaticpotential image is produced on the surface of target area I6 which iseffective, in conjunction with the conducting lm 26, to cause varyingdegrees of orientation of the particles 24 suspended in the liquid 20.Light projected against the surface I6, may, therefore, be modulatedthroughout its cross-sectional area with the result that an opticalimage may be projected on the viewing screen 32.

From the foregoing it may be seen, therefore, that an electron lightvalve may be controlled by two electron gun structures and theirproduced cathode ray beams, so that the surface of the target electrodemay be caused to vary within predetermined limits, even though thecathodes of the two electron gun structures are maintained atapproximately the same potential.

If the potential deviation limits between 11. and o do not aiord thenecessary differential potential to produce the desired result, thencurve 66 may be shifted to the right relative to curve 64 to therebyincrease the potential deviation limits by merely making the potentialsof the cathode (and associated electrodes) of electron gun structure Anegative with respect to the corresponding electrodes of electron gunstructure B. When this is done, a combination of effects will result,including the operation of the device as described in connection withFigure 2, as well as the operation of the device as described inconnection with Figure 3. Under these circumstances the 12 diierentSecondary electron response characteristics are brought about by reasonof both a beam vlocity variation and an angle of incidence varia ion.

It is, therefore, possible to exercise the present invention by relyingupon either a difference in beam velocity or a difference in theincidence angle of the beams, or a combination of these two variablesmay be employed at the same time.

Although the present invention is described as being particularlyapplicable for the reproduction of television images, such a system mayalso be used for .the production of other images in response topotential variations. Furthermore, the present invention is described asapplicable to a light valve using a suspension liquid and a plurality ofopaque platelets contained therein, but it is to be understood that thepresent invention may also be applied to other types of light valveswhere the light transmitting efficiency, the opacity or transparency ofa medium is controlled or varied by potential means or by anelectrostatic eld. In this respect the present invention may be used inconnection with a crystal mosaic operating to rotate the plane ofpolarization of polarized light in a manner shown and described in VonArdenne patents, Nos. 2,276,359 and 2,277,008 issued on March 17, 1943.

Naturally if the present invention is to be used in connection withtelevision, the horizontal and vertical deflection generatorsrepresented at 56 would operate under the control of synchronizingsignals, in order that the deflections of the cathode ray beams may bemaintained in synchronism with the television transmitter, and in orderthat their phase of operation may be properly maintained.

Various other alterations and modifications may be made in the presentinvention without departing from the spirit and scope thereof, and it isdesired that any and all such alterations and modiiications beconsidered Within the purview of the present invention, except aslimited by the hereinafter appended claims.

Having now described our invention, what we claim as new and desire tohave protected by Letters Patent is:

1. An electronic valve comprising a medium whose light transmittingcharacteristics are altered in response to a potential change comprisinga target area associated with said medium, a pair of electron gunstructures for developing independent cathode ray beams of substantiallyidentical beam velocities, means for varying the current intensity ofone of the developed beams, means for scanning the target area by saidone cathode ray beam at such an angle of incidence that the number ofproduced secondary electrons will be less than the number of arrivingbeam electrons thereby to cause a variable change in the potential ofthe surface of the target area in a negative direction, and mean forsubsequently scanning the target area by the second cathode ray beam atan increased angle of incidence that the number of produced secondaryelectrons will 'ebe in excess of the number of arriving beam electronsthereby to remove the potential established by said one beam and toreturn the target area to a datum potential level.

2. An electronic light valve for use in production of television imageswhich includes a. medium the transparency of which is altered by achange in an electrostatic field impressed thereon comprising a pair ofelectrodes associated with the medium, means for maintaining one of theelectrodes at a substantially uniform and fixed potential, the otherelectrode functioning as a target surface having high lateralresistivity, a pair of gun structures for developing individual focusedcathode ray beams, of substantially identical predetermined velocity,means for scanning the target surface in substantially bilateraldirections by one of the cathode ray beams, the angle of incidence ofthe one cathode ray beam at its predetermined velocity' being such thatthe number of secondary electrons produced at the target surface is lessthan the number of arriving primary electrons, means for modulating thecurrent intensity of the first cathode ray beam by television imagesignalsduring the scanning operation so that the potential of the targetsurface is altered in a negative direction -by an amount determined inaccordance with the current modulations of the cathode ray beam toproduce a-variable electrostatic eld across said medium, means forsimultaneously scanning the target area in substantially bilateraldirections by the other cathode ray beam, the angle of incidence of theother cathode ray beam being greater than the angle of incidence of saidone cathode ray beam so that at its predetermined velocity the number ofsecondary electrons produced at the target surface is greater than thenumber of arriving primary electronsk in order that the potential of thetarget surface may be altered in a positive direction to a predetermineddatum level thereby to remove the electrostatic eld produced by thescanning operation of said one beam the scanning operation of the twocathode ray beams being displaced, in point of time, by a predeterminedamount.

3. In an electronic light valve comprising a tube having a double endwall, a-suspension of light intercepting particles in a liquid vmediumbetween said double end Wall, electronic means to vary the lighttransmitting properties of said suspension in accordance with signalvariations, said means including an electron gun structure fordeveloping a focused cathode ray beam having a predetermined velocity,means for scanning the surface of one of the walls bythe developedcathode ray beam, the angle of incidence of the scanning cathode raybeam at its predetermined velocity being such that the secondaryelectron emission during the scanning action has a ratio less thanunity, means for modulating the scanning cathode ray beam by the signalvariations whereby the potential of the surface of the scanned Wall maybe altered in a negative direction by an amount determined by currentintensity of the modulated beam, means for electronically returning thepotential of the surface of the wall to a predetermined datum potentialleve1 comprising a second electron gun structure for developing a secondfocused cathode ray e beam having a velocity substantially identical tothe predetermined velocity of the first cathode ray beam, and means forsimultaneously scan-` ning the wall by the second cathode ray beam, theangle of incidence of saidV second cathode. ray beam being greater thanthe angle of incidence of the first cathode ray beam and being so chosenthat at the predetermined Ibeam velocity the secondary electron emissionduring such scanning action has a ratio greater than unity whereby thepotentialof the surface of the wall may be altered in a positivedirection to thereby cancel thepreviously produced negativepotential'change.

` liquid medium.

4. In an electronic light valve for producing television imagescomprising a tube having a double end wall, a suspension of lightintercepting particles in` a liquid medium between said double end wall,electronic means to vary the light transmitting properties of saidsuspension in accordance with received television image signals, saidmeans including an electron gun structure for developing a focusedcathode ray beam, means for scanning the surface of one of the walls bythe developed cathode ray beam, the angle of incidence of the scanningcathode ray beam being such that the secondary electron emission ratiois less than unity, means for current intensity modulating the scanningcathode ray beam by the television image signals whereby the potentialof the surface of the scanned wall may be altered in a negativedirectionby an amount determined by the beam current intensity, means forreturning the potential of the surface of the scanned wall to apredetermined datum level comprising a second electron gun structure fordeveloping a second focused cathode vray beam having a velocitysubstantially identical to that of the rst cathode ray beam, means forsimultaneously scanning the said wall by the second cathode ray beam,the yangie of incidence of the second cathode ray beam being such thatthe secondary electron emission lratio is greater than unity whereby-the potential of the surface of the scanned wall may be altered in apositive direction thereby to cancel theeiiects produced by the scanningoperation of the modulated cathode ray beam, and means for projectinglight through the 5. An electronic-:light valve for reproducingtelevision images comprising a tube having a double end wall, a liquidmedium'positioned between the double end wall, .said liquid-mediumsuspending a multiplicity of light intercepting particles theorientation of which are responsive tothe presence and intensity of anelectrostatic field, one of the walls including a target area having lowlateral conductivitma pair of electron gun structures for developingindividual focused cathode ray beams of substantially identicalvelocity, means for current modulating one of the cathode ray beams byreceived television signals,

means for scanning the surface of the target area by the modulatedcathode ray beam at such an angle that the secondary electron emissionratio of the surface of the target area is less than unity or theparticular beam velocity whereby the potential of the surface of thetarget area may be altered in a negative direction in accordance withthe current modulations of the scanning cathode ray :beam thereby toproduce an electrostatic charge image on the surface to eectdiiferentdegrees of orientation of the particles, means to projectsubstantially parallel light rays through the liquidmedium in order thatthe transmission of the light rays through the medium may be altered inaccordance with the orientation of the particles, means for subsequentlyscanning the surface of the target areal by the other cathode ray beamat such an angle that the secondary electron emission ratio of thesuraltered in a positive direction to remove the effects produced by thescanning operation of the modulated cathode ray beam.

6. In an electronic light valve wherein a medium is provided whose lighttransmitting emciency is altered in response to avarlatlon in an appliedelectrostatic field, the method of altering the potential of one of twoelectrodes for 'producing the electrostatic field which comprises thesteps of generating a first focused cathode ray beam, modulating thecurrent intensity of the generated cathode ray beam by signalpotentials, scanning the electrode by the current modulated cathode raybeam, the angle of incidence of the scanning cathode ray beam being suchthat the number of produced secondary electrons will be less than thenumber of arriving primary electrons thereby to alter the potential ofthe surface of the electrode in a negative direction in accordance withthe current modulations of the scanning beam, developing a secondfocused cathode ray beam of substantially constant current intensityhaving a velocity substantially identical to that of the first generatedcathode ray beam, and simultaneously scanning the electrode by thesecond developed cathode ray beam, the scanning operation by the secondcathode ray beam being subsequent in point of time, to the scanningoperation by the first cathode ray beam, the angle of incidence of thesecond cathode ray beam being such that the number of produced secondaryelectrons will be greater than the number of arriving primary electronsso that the potential of the surface of the scanned electrode is alteredin a positive direction thereby to eliminate the effects of the firstscanning operation by the modulated cathode ray beam.

'7. In an electronic light valve wherein a medium is provided whoselight 'transmitting characteristics are altered in response to avariation in an applied electrostatic field, the method 16 of alteringthe potential of one of two electrodes associated with the medium forproducing the electrostatic field which comprises the steps ofgenerating a first focused cathode ray beam, modulating the currentintensity of the generated cathode ray beam by television image signals,scanning the electrode by the current modulated cathode ray beam. theangle of incidence of the scanning cathode ray ibeam being such that thesecondary electron emission ratio is less than unity thereby to alterthe potential of the surface of the scanned electrode in a negativedirection in accordance with the current modulations of the beam,developing a second focused cathode ray beam of substantially constantcurrent intensity and having a velocity substantially identical to thatof the first generated cathode ray beam, simultaneously scanning theelectrode by the second developed cathode ray beam, the second scanningoperation being phase displaced from the first scanning operation by apredetermined amount, the angle of incidence of the second cathode raybeam being greater than the angle of incidence of the first cathode raybeam with the result that the secondary electron emission is greaterthan unity so that the potential of the surface of the scanned electrodeis altered in a positive direction to a predetermined datum levelthereby to remove the effects of the first scanning operation by themodulated cathode ray beam, and projecting light rays through the mediumin a direction parallel to the produced electrostatic field.

DAVID B. LANGMUIR.

JOHN S. DONAL, JR.

